The surface acoustic wave (SAW) phenomenon has been used to implement filtering in electronic applications. SAW filters include a piezoelectric substrate such as quartz or gallium arsenide (GaAs) and interdigitated transducers formed by photolithographic patterning of a thin metal layer. Application of an alternating voltage to the input transducer generates an alternating strain field that launches a surface acoustic wave that travels along the substrate surface before being converted back into an electrical signal by the output transducer. The velocity and attenuation of the propagating wave are very sensitive to properties, such as mass and viscoelasticity, of thin films formed on the device surface.
SAW filters are used in various applications such as mobile telephones, and provide significant advantages in performance, cost and size over other filter technologies such as quartz crystals (based on bulk waves), LC filters and waveguide filters. The surface of a SAW filter must be kept free of contacts and foreign materials to ensure surface waves are generated freely according to the incoming radio frequency. An air gap of at least a few microns is typically needed for SAW filters to function properly. Conventional SAW filters include a separator such as a carrier or liquid cell based on photo-definable epoxy SU-8 on the piezoelectric substrate. A lid is bonded to the separator to protect the separator during subsequent processing such as molding. The separator ensures a sufficient air gap exists between the surface of the SAW filter and the lid, keeping the SAW filter surface free of contacts and foreign materials.
However, the separator and lid increase the overall cost of SAW filter packages. The processing cost involved with placing the separator on the SAW filter, preparing the lid and bonding the lid to the separator further adds to the overall cost. The combined thickness of the separator and lid is typically at least 200 μm, substantially increasing the thickness of the final SAW filter package which is disadvantageous considering substantial industry pressure to reduce the size of packaged components. In addition, only a small space is available between the edge of the separator and the edge of the SAW filter die for attaching wire bonds. Reverse bonding techniques are often required to attach the wire bonds in this case. Reverse bonding techniques are not well adapted for Cu bond wires and also add to the complexity of the wire bond process.